The present invention relates to a method of counting individual particles in time-of-flight mass spectrometry for secondary ions or particles occurring at a high repetition rate after pulsed ion bombardment of a sample, with a detector being operated to measure the time-of-flight of the secondary particles in such a manner that the magnitude of the detector signal becomes independent of the number of simultaneously impinging secondary particles and all detector signals detected after an individual ion bombardment are recorded by a recording device.
In time-of-flight mass spectrometry, the important measuring task is a precise measurement of the time-of-flight of secondary particles between a time t.sub.0 at which they are generated and a time t.sub.i at which they impinge on the detector. The elapsed time between the times t.sub.i and t.sub.0 is a function of the particle's m/z ratio, where m represents the particle mass. If z is 1, the dependence of the particle's velocity is essentially on its mass. The frequency distribution of the time-of-flight of the secondary particles with respect to the elapsed time is a representation of the mass spectrum.
In time-of-flight mass spectrometry, essentially two different methods are employed to record the arrival times of the secondary particles, and these methods are discussed hereunder.
An article entitled "Recent Developments in Techniques Utilising Time-of-Flight Mass Spectrometry" by D. Price and G. J. Milnes, in Int. J. Mass Spectrom. Ion Proc. 60 (1984), incorporated by reference in the present application, discloses an analog recording method for recording arrival times of secondary particles. In this analog recording method, the particle detector which is employed operates as a linear amplifier. The amplitude of the detector signal is here proportional to the number of particles impinging on the detector per unit time. A fast transient recorder which is started by a signal correlated with the starting time t.sub.0 of a starting event--which may be the detection of a fission fragment--records the detector signal. The "stop" event in this arrangement can be the detection of a secondary ion whose creation involves the generation of the fission fragment mentioned above. In this apparatus, a signal amplitude as well as an associated elapsed time is digitalized in successive short time intervals and is stored. After the starting event, a complete time-of-flight spectrum is customarily recorded and stored in the memory of the fast transient recorder.
An article entitled "Cf-Plasma Desorption Time-of-Flight Mass Spectrometry" by R. D. Macfarlane and D. T. Torgerson, in Int. J. Mass Spectrom. Ion Phys. 21 (1976), incorporated by reference in the present application, discloses the counting of individual particles. In this article a detector is discussed which is operated in saturation and which generates a signal which is independent of the number of secondary ions or particles impinging on the detector per unit time. A subsequent fast discriminator emits a unit pulse per stop event to a stop input of a time/digital converter TDC. By summing up the stop events of several measuring cycles each actuated by the start event at the starting time t.sub.0, high signal to noise ratios can be realized.
In the analog recording method, the signal to noise ratio is limited to about two orders of magnitude because of the noise of the analog signals and the low amplitude resolution of the transient recorders.
In the individual particle counting method, the number of processible stop times per start recording cycle is limited. Moreover, stop events arriving within the relatively long dead time of about 50 ns after an event are not recorded.